1. Field of the Invention
The present invention relates generally to communication systems, and more particularly to a method, system, and apparatus for mitigating signal interruptions between a wireless communication device and a source, such as a satellite.
2. Background
Various satellite communication systems have been developed over the years. One early system architecture is referred to as Time Division Multiple Access (TDMA) and is characterized by assignment of time slots in a communication channel to each of a plurality of terminals, and with communication with the terminals taking place in the specifically assigned time slots. An improved system architecture is referred to as Code Division Multiple Access (CDMA). CDMA based communication systems generally provide greater bandwidth efficiency than do TDMA based communication systems.
Wireless communication systems are widely deployed to provide various types of communication such as voice and data. A typical wireless data system, or network, provides multiple users access to one or more shared resources. A system may use a variety of multiple access techniques such as frequency division multiplexing (FDM), time division multiplexing (TDM), code division multiplexing (CDM), and others. Examples of wireless networks include cellular-based data systems. The following are several such examples of cellular-based data systems: (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard), and (4) the high data rate (HDR) system that conforms to the TIA/EIA/IS-856 standard (the IS-856 standard). The HDR system is implemented in the CDMA communication standard 1xEv-DO.
In general, signal strength is a critical factor in establishing a communication link with good quality of service (QoS). In the case of satellite communications, in which the satellite appears as a point signal source of limited strength to a wireless communication device (WCD), and in which one satellite serves many subscribers across a wide geographic area, a clear line-of-sight between a WCD and a satellite signal source is usually required, and structures between a satellite and a receiver may cause an interruption. Note that this is true not only for satellite communications, but for many other wireless communication forms in which a direct transmission to a receiver is relied upon for signal reception, and in which diffused transmissions or reflections significantly affecting signal path are not generally useful.
There may be many obstacles between a satellite transmitter and a WCD such as a portable communication device, or a satellite radio receiver installed in a car or other vehicle. In satellite broadcast systems, there is typically no feedback mechanism from the receiver back to the satellite to request retransmission of lost packets. In other words, if one specific terminal, such as a receiver on a car, moves under an overpass or behind a building, it will lose packets broadcast from the satellite during the time interval that the car is shadowed by the overpass or the building. This event, called “shadowing” or “blocking”, will result in major degradation to the service being received by the mobile ground terminal.
A technique that has been used to mitigate loss of service during shadowing is redundant retransmission of information from the satellite, delayed by a few seconds. Then, if the car happens to enter a shadow and loses the packets for a short period of time, it can wait and try to receive the second copy of the same information that is transmitted a few seconds later. Accordingly, the WCD outputs information with a brief delay. A large drawback to this technique is that the throughput is fully halved because the same data is transmitted twice. With one satellite typically serving multiple subscribers as described above, and with more subscribers daily demanding more streaming video and audio options, increased throughout is highly desirable.